Wavelength Selective Switch (WSS) devices have become important aspects of modern optical networks, such as Dense Wavelength Division Multiplexing (DWDM) reconfigurable optical networks. A WSS device can dynamically route, broadcast, block and attenuate all DWDM channels within a network node.
Referring to FIG. 1, a WSS device 101 comprises a common multi-channel optical input port 103 for receiving multi-channel optical signals 105, and N opposing multi-wavelength ports 107. Each DWDM wavelength input from the common input port 103 can be switched to any one of the N multi-wavelength output ports 107, independent of how all other wavelength channels are routed. Current WSS devices are 1×N devices, while N×M WSS devices are today the object of medium term research. In the meantime, 1×N WSS devices can be cascaded to form larger architectures, for example whereby N×N wavelength selective matrix devices can be built by interconnecting, back-to-back, several 1×N WSS devices.
The wavelength switching process can be dynamically changed through an electronic communication control interface on the WSS device. For example, a mechanism based on Variable Optical Attenuation (VOA) operates in a WSS device for controlling each wavelength. As such, each wavelength can be independently attenuated for channel power control and equalization, or completely blocked.
Because every wavelength in the 1×N WSS device can be switched to any one of the N output ports 107, this switch can be used in a fully flexible Reconfigurable Optical Add Drop Multiplexer (ROADM) with multiple optical ports, each of which carries multiple wavelengths, coupled with respective add/drop sections where the client traffic is added/dropped in the node.
Fast ReRoute (FRR), also called local restoration, is a local restoration network resiliency mechanism. In a one-to-one Fast ReRoute for a link protection recovery technique, a Label Switched Path (LSP) passing through a link is protected by a backup path which originates at the node immediately upstream to that link. This local mechanism provides faster recovery because the decision concerning recovery is strictly local and does not involve the overall node chain. This recovery scheme has been defined and specified only for packet networks until now, as defined in Internet Engineering Task Force (IETF) RFC 4090. In the current field of Wavelength Switched Optical Networks (WSON), a practical implementation of a Fast ReRroute mechanism is prevented due to the time required for power leveling (or power equalization) operations that need to be carried out after a rerouting procedure, as will be explained below.
The power of a transmitter line amplifier is given by a fixed relationship between the number of active channels and the profile of the amplifier. The output power of the transmitter amplifier is given by the type of profile and the total number (M) of channels present in the line. M is given by the sum of already present channels (M−1) and the number of channels that have been added (usually one at a time).
Every time a new channel is activated or switched off in transmission, a new power leveling or equalization procedure is required. In particular, there is the need to adjust both the total power and the per-channel power, and this implies a set of subsequent leveling adjustment that lead to a long set-up time.
FIGS. 2a and 2b show how a FRR procedure would be implemented according to known techniques. Referring to FIG. 2a, when transmitting traffic from Node A to Node D, (i.e. along links 2131, 2132, 2133, 2134 and 2135), a node along this path, for example Node B, is shown as having a duplication of traffic towards Node C and Node E. When the link 2133 between Node B and Node C is operating correctly, the traffic directed to Node E is blocked by a mechanism 205 based on Variable Optical Attenuation (VOA) in Node B (shown as a shaded rectangle). Therefore, in this mode, the flow of traffic is shown by the dotted lines, whereby the main flow of traffic flows from Node A to Node D along the links 2131, 2132, 2133, 2134 and 2135, and wherein the flow of duplicated traffic flows along the links 2131, 2136. As such, the flow of duplicate or backup traffic is blocked at Node B using the VOA 205.
Referring to FIG. 2b, when a failure occurs in the link 2133, the VOA 205 is tuned to feed the traffic to Node C via Node E, (i.e. along the links 2137, 2138 and 2139), thereby bypassing the failed line 2133 with a local detour via Node E. The flow of traffic is shown by the dotted lines, whereby the main flow of traffic flows from Node A to Node B along the links 2131, 2132, but stops at the VOA 203 because of the failure of the link 2133. The duplicated or backup traffic flows from Node A to Node D via the detour from Node B to Node E to Node C, i.e. along the links 2136, 2137, 2138, 2139 and 21310).
It can be noted that the transmitter side of Node B, along the line B-E (i.e. path 2137) is involved with the addition of one new channel. This requires power leveling adjustments in Nodes B and E.
This can be appreciated from FIGS. 3a and 3b, which show further details of a typical node, such as Node B of FIGS. 2a and 2b. 
A ROADM node comprises two or more DWDM ports. In the example of FIG. 3a, a ROADM node 301 is shown as comprising four ports, which are labeled north, south, east and west. Each port, as shown in further detail in FIG. 3b, comprises a wavelength selective switch 303. Each wavelength selective switch is coupled to an add/drop multiplexer 305 that is adapted to add or drop a wavelength channel. An operational amplifier unit 307 comprises transmit and receive amplifiers. A power management unit 309 monitors the power level of a transmitter (and/or a receiver although not shown), to determine whether any channels are present in the outgoing DWDM flow. When a channel is added or dropped, power leveling or equalization must be performed, which causes an undesirable delay.
Prior to the introduction of a Generalized Multi-Protocol Label Switching (GMPLS) control plane in photonic networks, new channels were added manually, and as such there was no need to consider the addition or deletion of more than one channel contemporaneously. However, in a GMPLS controlled network it is quite common to have more than one LSP setup or tear down requests at the same time.
Processing such requests in a serial manner has the disadvantage of causing unacceptable delays and traffic loss. However, on the other hand, processing a high number of setup or tear down requests at the same time can cause significant power peaks (positive or negative) and therefore cause service disruptions to the LSPs already in place and carrying traffic.